Mass spectrometry is a widely used analytical technique that measures the mass-to-charge ratio of charged particles from a sample chemical compound. Mass spectrometry has many applications such as determining particle mass, the elemental composition of a sample or molecule, and the chemical structures of molecules, such as peptides and other chemical compounds. In the mass spectrometry process, chemical compounds are ionized to generate charged molecules or molecule fragments and measuring their mass-to-charge ratios.
In a typical mass spectrometry system, a sample is loaded onto the mass spectrometer and undergoes vaporization. The components of the sample are ionized by one of a variety of methods, such as exposure to an electron beam, which results in the formation of ions. The ions are separated according to their mass-to-charge ratio in an analyzer by electromagnetic fields.
A mass spectrometer includes an ion source, a mass analyzer and an electron detector. The ion source converts gas phase sample molecules into ions. The mass analyzer sorts the ions by their masses by applying electromagnetic fields. The electron detector measures the value of an indicator quantity and thus provides data for calculating the abundances of each ion present.
A typical electron detector includes an electron multiplier that is a cylindrical tube having a cathode at one end and an anode at the opposite end. The ions are injected at the cathode and an electrical voltage is applied between the cathode and the anode resulting in the ions being exciting and colliding with the interior surface of the cylinder to produce electrons. The collisions create an avalanche of electrons which exit a hole at the opposite end of cylinder and are collected by the anode. An electrometer is connected to the anode to measure the resulting current. The amount of electrons produced is measured by electron gain which is a function of the voltage between the cathode and the emitter nodes. The higher the applied voltage, the more energetic the electrons from the ions are thereby producing more electrons, and therefore the more the gain increases.
Conventional electron detectors set the voltage at a fixed value and gain is constant over time. The user may calibrate the electron gain and determine the current at the anode. It is desirable to adjust the voltage over a dynamic range to create a larger signal for measurement purposes. Different ranges of voltages are desirable since the ion strength of the measured compounds changes with the type of compound. The signal ranges are limited in a fixed voltage because of the drift (lower limit) and a high limit set by the saturation point from a certain voltage.
Present electron detectors provide dynamic range by determining the highest peak of a signal and for the next scan the electron multiplier is set to an appropriate gain with the peak value. This produces a large range for the detector, but when the voltage between the cathode and anode changes quickly in such dynamic ranging, a transient error signal is introduced into the electrometer through capacitive coupling to its electron collector. A common configuration of electron multipliers is have a bias resistor at the anode (emitter) end that generates, as a result of the channel current flowing through the resistor, a bias voltage for attracting the exiting electrons to the electrometer collector. Since the channel resistance and bias resistor form a voltage divider, changes in the multiplier voltage also result in changes to the bias voltage. Additionally, capacitive coupling through the channel body, from cathode to emitter, also perturbs the emitter potential. Such bias changes may result in minor changes in collection efficiency, but more importantly, the emitter potential changes are capacitively coupled to the electrometer collector.
In order to solve such distortions, a zener diode has been connected in place of the integral resistor, which stabilizes the voltage and reduces the error. However zener diodes still have problems that cause additional distortion. Zener diodes do not provide perfect voltage regulation and cause slight voltage changes when the current through the zener diode changes. Further, relatively high voltage zener diodes are required to ensure efficient electron collection by the electrometer. In avalanche mode, zener diodes produce diode noise thereby adding noise to the output signal of the electron multiplier.
Therefore it would be desirable to have an electron detector that is capable of dynamic range without distortions from current produced from the parasitic resistance in the electron multiplier tube channel in conjunction with the applied voltage. It would be desirable for a circuit that may correct for distortions that may be integrated with the mechanical components of the electron detector. It would also be desirable to have a circuit to compensate for the transient current generated by capacitive coupling between the cathode and anode electrodes when the channel voltage changes dynamically. It would also be desirable to have a circuit to compensate for distortions from an avalanche voltage for a zener diode used to compensate for voltage distortions. It would be desirable to have a compensation circuit that is small enough to be included in the vacuum around certain parts of the electron multiplier.